Arsenic, in its metallic form, does not occur in nature and in fact is practically of no commercial value. Arsenic trioxide, however, is a classic inorganic poison, which was used for many years to, among other things, control insects. Although some arsenic enters the environment from manmade sources, most arsenic contamination is naturally occurring. Arsenic in water is almost always anionic, and generally takes on one of two forms, either the “trivalent” arsenite anion or the “pentavalent” arsenate anion. The terms trivalent and pentavalent refer to the valence of the arsenic in the arsenite and arsenate. Arsenate is generally considered much easier to remove than arsenite.
In recent years, the presence of dissolved arsenic, as well as other contaminants, in groundwater has emerged as a major concern on a global scale. The concern stems, at least in part, from the fact that groundwater is a major source of potable water. According to the estimate of the United States Environmental Protection Agency (USEPA), the newly promulgated 10 ug/L arsenic maximum contaminant level (MCL) in drinking water would require corrective action for more than 4000 water supply systems serving approximately 20 million people. A vast majority of these systems are groundwater systems. Natural geochemical contamination through soil leaching is the primary contributor of dissolved arsenic in ground waters around the world.
On the other hand, recovered arsenic is an important commercial commodity, with about 70% of the end uses being wood preservatives and herbicides, while cotton desiccants, glass and flotation reagents account for some of the other uses. The major raw material used in these arsenic chemicals is arsenic trioxide, the world demand for which is estimated to be about 100,000 tons per year (as As). Practically all of this arsenic oxide is recovered from mineral processing residues, mostly from the flue dusts produced in the smelting of sulfide concentrates.
The removal and recovery of arsenic, as well as other metals and contaminants, from, for example, process solutions process solutions, effluents and aqueous solutions is thus an important process. Generally, there are several known methods for removing and recovering metals and contaminants including precipitation, co-precipitation, adsorption, liquid-liquid extraction and ion exchange. Traditional treatment methods for the removal of, for example, arsenic from water include coagulation in the presence of an iron salt, adsorption by activated alumina exchange by a strongly basic anion resin, and various membrane processes, such as reverse osmosis and electro-deionization. Methods of coagulation and filtration are also well documented.
All of the traditional treatment methods for the removal of metals and contaminants from, for example, process solutions, effluents and aqueous solutions have had varying success. For example, when properly constructed and operated, oxidation followed by iron coagulation and filtration can reduce as much as 95% of arsenic that may be present Other metals, such as aluminum and magnesium to name only a couple, can similarly remove arsenic. However, with any coagulation process, if upsets occur, the percentage of removal can be somewhat problematic. There is also the necessity to add iron salt if insufficient iron is present naturally and to deal with arsenic laden sludge generated by the process. Also, membrane processes are generally considered overly expensive for drinking water applications, unless reduction of total dissolved solids is desired. On the other hand, when membrane processes are employed for other reasons, their ability to remove arsenic provides an additional benefit. Ion exchange, although often touted for arsenic removal is limited, primarily because sulfate, which is present in most potable water, interferes strongly. Additionally, there are numerous inorganic adsorbent materials and medias that have demonstrated various degrees of success at removing metals from process solutions, effluents and aqueous solutions, for example.
Chanda et al. authored “Ligand Exchange Sorption Of Arsenate And Arsenite Anions By Chelating Resins In Ferric Ion Form: I. Weak-Base Chelating Resin Dow XFS-4195,” Reactive Polymers, 8(1988) 251-261. Chanda et al. describe a weak base chelating resin which is activated by treatment with hydrochloric acid solution after which the resin is agitated in a solution of FeCl3—H2O then rinsed with water. The resulting ferric polychelate resin is used to remove arsenic until exhausted and regenerated treated NaOH, washed and then protonated afresh with an acidified ferric chloride solution, rinsing with water and returning to service.
The Chanda et al. publication describes loading iron as a cation (Fe+3) by chelation, not by ion exchange. The resin involved is functionalized with a chelating agent that is also an amine. The interaction with the resin and iron is by ligand bonding. The complex amine functionality also has weak base anion exchange characteristics but no anion exchange is involved in the reaction with iron. Since the resin is weakly basic, there is no electrostatic repulsion mechanism to prevent the positively charged iron from entering and bonding with the chelating groups. Also, the iron is loaded from dilute acidic solution which favors the Fe+3 and avoids forming the complex FeCl4− which is the opposite of the conditions used in the present invention where the iron is loaded as the FeCl4− anionic complex. The fact that Chanda et al. rinse the iron laden resin with water as a final step also indicates that the iron is not in the form of an anionic complex. If it were such a complex, the complex would decompose and leave the resin free of iron.
Thus, Chanda et al. maintain iron in the Fe+3 form (as a cation) and operate under acid conditions to stabilize the ligand bond. Regeneration of arsenic laden chelating resin is by contact with NaOH. However, this precipitates iron, which destroys the functionality of the resin. In order to reuse the resin, Chanda et al. must reload the iron by passing an acidified dilute ferric chloride solution, which solubilizes and removes the precipitated iron from the previous cycle. Some build up of iron is observed, primarily on the surface, which eventually causes a decrease in performance after a few cycles. Chanda et al. note this and describe a step of completely stripping the iron, including the precipitated iron, by rinsing the resin with dilute hydrochloric acid and starting anew.
In contrast, the present invention uses strongly basic resins, loads the iron from highly concentrated solutions with very high chloride salt concentrations specifically designed to form the FeCl4− and to load the entire resin with the complex by ion exchange as an anion. The present invention also uses NaOH to precipitate the iron inside the gel phase of the resin. When the arsenic laden resin is regenerated to desorb the arsenic with NaOH, the iron remains unaffected. The alkaline regeneration process has no impact on the iron content of the resin because it is immobilized as a precipitate inside the resin and is insoluble in NaOH.
U.S. Pat. Nos. 4,116,856, 4,116,857, 4,116,858, 4,159,311, 4,183,900, 4,243,555 and 4,347,327 of Lee et al. describe anion exchange resins having suspended therein microcrystalline LiOH-2Al (OH)3 and MgX2-2Al(OH)3 structures for recovering lithium and magnesium ions, respectively, from brines. Lee et al. do not describe loading AlCl3 on a resin. Instead, the resin is soaked in an AlCl3 solution and then treated with a dilute solution of ammonium chloride and ammonium hydroxide to convert the aluminum to Al(OH)3 which coats the surface of the resin. There is no complex anion formation. There is also no indication that aluminum undergoes any complex formation or is attracted by such a mechanism into an anion resin. The ammonia is sufficiently basic to precipitate the aluminum but not so basic as to re-dissolve it as an anion complex.
In order to create an anion complex from aluminum, one would have to raise the pH high enough to make the aluminum behave as an anionic complex-aluminate. This would be easy to do with a stronger alkali solution such as NaOH or KOH or pure NH4OH. The mixture of NH4Cl with NH40H lowers the pH so that the amphoteric state is avoided and aluminum stays on the surface. As such, it is impossible to migrate into the gel phase. Lee et al. confirm this stating that “small crystals formed in small pores, voids and spaces in the resin which are detectable by X-ray diffraction if not by microscope.” In other words, the metal is not inside the gel phase of the resin as in the present invention.
U.S. Pat. Nos. 4,366,261, 4,446,252 and 4,629,741 of Beale, Jr. describe anion exchange resins having chromium III oxide (Cr2O3-nH2O) in the resin for removing metal cations from aqueous solutions. In the Beale patents, resin is soaked in a saturated solution of chromium chloride that is mixed with hydrated chromium chloride. The chromium is not able to load onto the anion resin as a chloride complex and it does not form anionic complexes with chloride. It is in the form of Cr+3, CrCl3 (dissociated) or as CrCl3 (solid) as part of a supersaturated solution or slurry. After the soak, the solution is drained off by filtration, and the wet resin is dried. Clearly, the Beale patents describe a process for coating a resin surface with a foreign substance. However, it is unclear whether an anion resin is even required. Whether an inert polymer or a cation resin would perform as the substrate as well as an anion resin is not obvious to one of ordinary skill in the art. Instead, Beale points out the order of preference of the resin to be used is dependent upon the physical porosity of the resin. There is no mention of ion exchange capacity which clearly means that the physical surface area, not ion exchange capacity, is the most important factor as dictated by a process of surface coating.
All types of anion exchange materials used in, for example, adsorption columns that operate for many thousands of bed volumes are prone to fouling with suspended solids and operational problems, such as channeling. Since many types of anionic exchange materials are able to reduce arsenic below 10 ppb, the materials that cost the least per pound are often favored by equipment suppliers. However, cost per pound is not always the best indicator of the effectiveness of an anion exchange material. As the date for implementing the newly promulgated 10 ug/L arsenic MCL in drinking water looms ever closer, available anionic exchange materials will come under closer scrutiny. Similarly, the maximum contamination levels of other contaminants will also require more efficient and improved anion exchange materials. Therefore, there is a need for improved anion exchange materials, improved methods of making anion exchange materials and improved methods of removing metals and contaminants from process solutions, effluents and aqueous solutions in general.